#include <iostream>
#include <vector>
#include <unordered_map>
using namespace std;

class Solution
{
public:
    int numberOfArithmeticSlices(vector<int>& nums)
    {
        int n = nums.size();
        unordered_map<long long, vector<int>> hash;
        for (int i = 0; i < n; i++) hash[nums[i]].push_back(i);

        int sum = 0;
        vector<vector<int>> dp(n, vector<int>(n));
        for (int j = 2; j < n; j++)
        {
            for (int i = 1; i < j; i++)
            {
                long long a = (long long)2 * nums[i] - nums[j];
                if (hash.count(a))
                    for (auto k : hash[a])
                        if (k < i) dp[i][j] += dp[k][i] + 1;
                sum += dp[i][j];
            }
        }
        return sum;
    }
};

void Test01()
{

}

int main()
{
    Test01();
    return 0;
}